Method of forming a precision glass mold and article

ABSTRACT

A method of forming precision glass molds suitable for molding glass optical elements or lenses is disclosed. The glass molds define first and second opposed optical molding surfaces. Each master is formed by defining a master cavity adapted to form a first glass mold. A quantity of glass mold material is disposed within such cavity and molded in conformation with said master. The molding surface of each such glass mold defines an optical surface to be subsequently formed on an optical element.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is related in part to U.S. Pat. No. 4,481,023, issuedto Marechal and Maschmeyer on Nov. 6, 1984, assigned to the assignee ofthe present invention, and incorporated herein by reference.

This application is also related to the following copending applicationsfiled concurrently herewith, each of which are assigned to the assigneeof the present application, and incorporated herein by reference:

"Method of Molding Glass Optical Elements and Article", by L L.Carpenter, R. J. Hagerty, R. 0. Maschmeyer, Mark L. Morrell, and P. A.Schrauth, Ser. No. 320,459, Filed March 8, 1989;

"Apparatus for Molding Glass Optical Elements", by L. L. Carpenter, M.L. Morrell, and P. A. Schrauth, Ser. No. 320,753, Filed March 8, 1989;and

"Apparatus for Molding Glass Molds", by L. L. Carpenter, Mark L.Morrell, and P. A. Schrauth, Ser. No. 320,693, Filed March 8, 1989.

BACKGROUND OF THE INVENTION

Manufacture of optical lens elements has long involved complex, slow,expensive and tedious procedures. Although concerted efforts to improveexisting methods and apparatus of manufacture have been undertaken,prior art methods still have many disadvantages and problems.

Precision optical elements require polished surfaces of exacting figureand surface quality. The surfaces demand fabrication in proper geometricrelation to each other and, where the elements are used in transmissionapplications, they will be prepared from a material of controlled,uniform, and isotropic refractive index. For some applicationsnon-isotropic refractive index materials have been known.

Precision optical elements of glass are customarily produced by means ofone of two complex, multi-step processes. In one process, a glass batchis melted in a conventional manner and the melt formed into a glass bodyhaving a controlled and homogeneous refractive index. Thereafter, thebody may be reformed utilizing well-known repressing techniques to yielda shape approximating that of the desired final article. The surfacefigure and finish of such an intermediate product are not suitable forimage forming optics. The intermediate article is fine annealed todevelop the proper refractive index, and the surface figure thereof isimproved by means of conventional grinding practices. Another methodinvolves forming a glass melt into a bulk body which is promptly fineannealed and subsequently cut and ground to articles of a desiredconfiguration.

Both of the preceding processes have similar limitations. The surfaceprofiles that are produced through grinding are normally restricted toconic sections, such as flats, spheres, and parabolas. It should beunderstood that other shapes, in particular, general aspheric surfacesare difficult to grind. In both processes, the ground optical surfacesare polished employing conventional, but complicated polishingtechniques which are intended to improve surface finish withoutcompromising the surface figure. In the case of the aspheric surfaces,such polishing requires highly skilled and very expensive hand-working.A final finishing operation, such as, for example, edging is alsocommonly required. Edging ensures that the optical and mechanical axesof a spherical lens coincide. Edging, however, does not improve therelationship of misaligned aspheric surfaces, if such are present, whichfactor accounts, at least in part, for the difficulty experienced ingrinding such lenses with precision.

Direct molding of lenses to a finished state could, at least inprinciple, eliminate the grinding, polishing and edging operations,which are especially difficult and time consuming for aspheric lenses.Such molding processes are employed for fabricating plastic lenses.However, existing plastics suitable for optical applications areavailable in a limited refractive index and dispersion range. Inaddition, many plastics scratch easily, display birefringence and areprone to the development of yellowing and haze. Abrasion resistant andanti-reflective coatings have been used but have failed to fully solvethese problems with plastics. Further, plastic optical elements aresubject to distortion from mechanical forces, humidity, and heat. Boththe volume and refractive index of plastics vary substantially withvariations in temperature thereby limiting the temperature interval overwhich plastics are useful.

The properties of glass render it generally superior to plastic foroptical applications. Conventional hot pressing of glass, however, doesnot provide the exacting surface figures and surface qualities demandedfor image forming optics. The presence of chill wrinkles in the surfaceand surface figure deviations are chronic problems. Similar problems canbe encountered in conventional repressing techniques as noted above.

Numerous means and devices have been employed to correct theshortcomings of conventional hot glass pressing processes andapparatuses. Among these are special pressing apparatuses utilizingisothermal pressing, i.e., pressing using heated molds and preheatedglass so that the temperatures under which the pressing step is carriedout vary only slightly across the glass preform during the pressinginterval, and using a gaseous environment inert to the glass and moldmaterials during the pressing operation. In addition, special materialsto construct the molds, special glass compositions and molding processparameters have been developed and used in an effort to improve thequality of lenses as well as other optical elements which are directlypressed.

Various patents related to mold and glass manufacture are noted anddescribed below, and all of these patents are hereby expresslyincorporated herein by reference. U.S. Pat. No. 2,410,616 describes anearly apparatus and method for molding glass lenses. The molds arecapable of being heated and the temperatures thereof controlled withinnarrow ranges compatible which the glasses being molded. An inert orreducing gas environment, preferably hydrogen, is used in contact withthe mold surfaces to inhibit oxidation thereof. A flame curtain,normally burning hydrogen, over the opening of a chamber enclosing themolds to prevent the entrance of air thereinto is described.

U.S. Pat. No. 3,833,347 is similarly directed to an apparatus and methodfor press molding glass lenses. The molds can be heated and thetemperature controlled. An inert gas surrounds the molds to precludeoxidation. This patent discloses the use of mold surfaces composed ofglass-like carbon which are distinguished from metal dies that werestated to produce lens surfaces not suitable for photographicapplications. The method described comprises eight steps includingplacing a chunk of glass into a mold, evacuating the chamber surroundingthe mold and introducing a gas therein, raising the mold temperature toabout the softening point of the glass, applying a load to the mold toshape the glass, reducing the temperature of the mold to below thetransformation temperature of the glass while maintaining the load onthe mold to prevent distortion of the shaped glass body, removing theload, cooling the mold to about 300° C. to inhibit oxidation of theglass-like carbon, and lastly opening the mold. This patent assertedthat lenses so produced were essentially strain-free without the needfor further annealing.

A similar teaching of an apparatus and method for transfer molding glasslenses employing glass-like carbon surfaces on the mold is found in U.S.Pat. No. 3,844,755. The use of mold coatings to enhance the surfacequality of the pressings, to improve mold durability, and act as aparting agent from the molten glass is suggested in U.S. Pat. No.3,244,497. This patent describes a lens blank molding apparatus whereina temperature controlled plunger and an insulated mold base offeringcontrollable heat transfer to a supporting press table are described.The apparatus, however, is designed for pressing relatively thin lensblanks, which factor is an important contributor to the temperaturecontrol attainable with the apparatus.

U.S. Pat. No. 4,481,023 describes an alternative molding apparatus fordirect pressing of lenses of optical quality. Temperature control of themolding surfaces is also provided, and the apparatus is designed forpressing at relatively high glass viscosities of 10⁸ -10¹² poises. Thiscorresponds to a relatively low pressing temperature, which helps toreduce difficulties stemming from nonuniform heat flow.

U.S Pat. No. 3,244,497, supra, teaches refractory coating selected fromthe group consisting of refractory nitrides, borides, carbides, andoxides. Coatings no thicker than approximately half of the wavelength ofvisible light, e.g. 0.5 microns, are suggested in order that the coatingfaithfully reproduce the mirror finish of the underlying mold surface.

U.S. Pat. No. 3,900,328 generally describes molding glass lensesutilizing molds fabricated from glass-like carbon. This referencediscloses placing a portion of heat softened glass into a cavity of amold prepared from glass-like carbon, applying appropriate amounts ofheat and pressure to the mold while maintaining a non-oxidizingatmosphere in the vicinity of the mold, cooling and opening the mold,and then removing the finished lens from the mold.

U.S. Pat. No. 4,168,961 describes a method for precision molding ofoptical glass elements wherein a mold having mold surfaces of a siliconecarbide/glassy carbon mixture is taught. The patent suggests thatelements molded employing such mold material exhibit high surfacequality and surface accuracy. However, molding while maintaining acontrolled atmosphere is required to avoid oxidation of this material, acondition which substantially reduces the practical economical value ofthe method.

Press forming optical lenses from hydrated glass is taught in U.S. Pat.No. 4,073,654. The process involves placing granules of hydrated glassinto a mold, drawing a vacuum on the mold, heating the mold to asufficiently high temperature to sinter the granules while the mold issealed to prevent escape of water vapor therefrom, applying a load tothe mold, releasing the load from the mold, and opening the mold.Glass-like carbon, tungsten carbide, and alloys of tungsten aresuggested mold materials.

European patent application No. 19342 discloses isothermal pressing ofglass lenses at temperatures above the softening points of the glassesemployed, i.e. at temperatures where the glasses exhibit viscosities ofless than 10⁷.6 poises

U.S. Pat. No. 4,139,677 teaches precision molding of optical glasselements in a mold having molding surfaces formed of silicon carbide orsilicon nitride. This method allegedly provides good surface quality andconfiguration, however, it requires maintaining an oxygen-freeatmosphere within the molding chamber to avoid oxidation of the moldcoatings.

U.S Pat. No. 4,747,864 describes glass optical elements formed by adirect molding process at glass viscosities in the range of 10⁸ -10¹²poises. Selected moldable alkali aluminofluorophosphate optical glassesare pressed to an optical surface finish in air utilizing an opticallysmooth titanium nitride molding surface, the surface being provided, forexample, as a surface coating on a stainless steel mold or on a nickelchromium alloy mold supporting an electroless nickel base coating.

U.S. Pat. No. 4,734,118 describes a mold for pressing a glass preformwhich has an overall geometry similar to the desired final lens. The topand bottom mold dies having mold cavities which match the configurationof the final lens. A glass preform s heated to the molding temperaturewhile the mold parts are separately heated. The mold parts are broughttogether against a ring having a thickness which governs the thicknessof the lens to be molded. The volume of glass that is put into themolding cavity is controlled by measuring its mass. The referenceteaches a variety of mold surface materials such as 400 series stainlesssteels, electroless nickel, beryllium nickel alloys, tungsten carbide,alloys of noble metals such as platinum, rhodium and gold and fusedsilica. The patent teaches that the mold material itself is not criticalbut must be capable of accepting a good surface finish.

In order to realize the economic advantages of employing direct moldingtechniques for products such as aspheric lenses, factors relating to theservice life of the molds employed for the pressing operation are mostsignificant and must be taken into strict account. The machining ofaspheric shapes in molds renders the molds relatively expensive,particularly since very hard and durable mold materials are generallyrequired. This is especially true for molding processes involvinglow-temperature, high-viscosity molding, since higher molding stressesare involved.

Primary factors affecting mold life include chemical reactions occurringbetween the hot mold and the glass to be molded, and between the hotmold and the atmosphere. The latter factor is particularly significantwhen rapid production rates that prohibit cooling of the shaped lens inthe mold are desired. Prior art approaches have suggested using acontrolled atmosphere for molding to avoid oxidation or otherdegradation of the mold surfaces, however, such steps are inconsistentwith rapid and economical optical element production.

The development of direct glass element molding has been substantiallyassisted by the discovery of new glass compositions which can be moldedat relatively low pressing temperatures while not being subject toattack by moisture in the manner usual for soft glasses. U.S. Pat. No.4,362,819 discloses examples of alkali aluminofluorophosphate glassesuseful for such applications. However, pressing of such glasses ateconomical rates has been difficult because of limited compatibilitybetween these glasses and conventional mold materials.

SUMMARY OF THE INVENTION

In view of the foregoing state of the art, it is an object of thepresent invention to provide a mold construction and a method of formingmolds which can be economically practiced and which provides long moldlife.

It is a further object of the present invention to provide a method ofmaking a high precision glass molds rapidly and economically.

It is a still further object of this invention to provide a method ofmolding glass molds suitable for molding precision optical elementsembodying small optical radii.

It is another object of this invention to provide precision glass moldssuitable for molding precision optical elements embodying small opticalradii.

It is still another object of the present invention to provide aneconomic and reproducible mold construction and molding process usefulfor molding aspheric glass elements and lenses with accurate surfacefigure and good surface finish.

It is still another object of the present invention to provide a glassmolds for molding glass optical elements economically and with greatprecision.

Briefly, according to the present invention, a mold assembly including apair of glass molds defining first and second opposed optical moldingsurfaces is formed as follows. A first master, defining a first mastercavity adapted to form a first glass mold is provided, within whichcavity a quantity of glass mold material is disposed. The first glassmold is then molded having a first glass molding surface adapted to forma first optical surface. The process is repeated with a second master soas to form a second glass mold having a second glass molding surfaceadapted to form a second optical surface.

The first and second glass molds thus formed are disposed withinconstraining means in an opposed operative relationship such that thefirst and second glass molding surfaces, at least in part, define aglass mold cavity having a predetermined volume, size and shape. Apredetermined quantity of glass optical element material is disposedwithin the glass mold cavity thus formed, and a glass optical elementhaving first and second optical surfaces is molded.

Precision inserts or rings having predetermined desired physicalcharacteristics may be employed in connection with defining the cavitiesfor forming the first and second glass molds as well as the glass moldcavity. Similarly, a master sleeve and a glass optical element sleeveare employed as constraining means to define the cavities.

To facilitate the formation of the glass molds, shaped and polishedelements or slugs are formed from which the first and second glass moldsare molded. Similarly, shaped and polished preforms are formed, such as,for example, spherical balls, from which the optical glass elements aremolded.

One glass mold formed in accordance with this invention and used formolding in connection with another mold formed of metal, glass ceramicor the like is also contemplated by this invention, as is the molding ofnon-glass optical elements or lenses.

These and additional objects, features and advantages of the presentinvention will become apparent to those skilled in the art from thefollowing detailed description and the accompanying drawing, which isincorporated in and constitutes part of the present specification, onwhich, by way of example, only the preferred embodiments of thisinvention are illustrated.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a cross-sectional elevation of an apparatus illustrating themethod of forming a glass mold in accordance with the present invention.

FIG. 2 is an front elevation of a slug or preform suitable for formingone of the glass molds of the present invention.

FIG. 3 is a cross-sectional view of one of the glass molds formed inaccordance with the present invention.

FIG. 4 is a front elevation of another slug or preform suitable forforming a glass mold in accordance with the present invention.

FIG. 5 is a cross-sectional elevation of another glass mold formed inaccordance with the present invention.

FIG. 6 is a cross-sectional elevation of an apparatus illustrating themethod of forming a glass optical element illustrating a method inaccordance with the present invention.

FIG. 7 is a cross-sectional view of a molded glass optical element inaccordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

It is to be noted that the drawings are illustrative and symbolic of thepresent invention, and there is no intention to indicate scale orrelative proportions of the various elements shown therein.

Referring to FIG. 1, there is shown a slug or preform 10 disposed withinmaster sleeve 12. In opposing cooperative relationship with slug 10 ismaster or mold element 14. Examples of materials suitable for formingmaster 14 are Inconel 718, stainless steel type 420, tungsten carbide,and the like.

The molding or optical surface of the master may degrade in use throughchemical attack, corrosion, denting, abrasion, adherence of the materialto be molded, or the like. To minimize such degradation, the mastersurfaces may be coated or plated. Suitable coating and plating materialsare electroless nickel (E1Ni) for Inconel 718 and type 420 stainlesssteel, and gold for tungsten carbide. Other possible coating and platingmaterials are hafnium, nitride, boron carbide, and amorphous diamond.

Referring again to FIG. 1, master 14 is shown formed with a snout 18which protrudes or extends outwardly from master 14, and has a desiredoptical prescription on its surface 16 when used to form molds foroptical elements or lenses. A master ring 20 is disposed over and aboutsnout 18 of the master in a manner so as to form the peripheral portionof the mold surface as will be hereinafter described in more detail. Thematerial of master ring 20 is preferably tungsten carbide but may alsobe Macor.sup.(®) glass ceramic or stainless steel.

Lens design data is used to calculate the profile of surface 16 ofmaster 14. The profile compensates for the different coefficients ofthermal expansion of the lens and mold materials at the formingtemperature to generate the required mold figure.

To design the profile of the master, it is necessary to start with theparameters of the ultimate optical element or lens to be formed. Forpurposes of this description, reference will be made to the molding of alens although this invention is not so limited. The specific material,size, shape, geometric configuration and the like of the lens must firstbe defined. At least in part, these will depend on the ultimate utilityof the lens.

The size and shape of the lens at its forming or molding temperature isfirst calculated taking the temperature coefficient of expansion andother relevant parameters of the lens and lens material into account.This provides the size and shape of the mold cavities and lens mold ringat the lens forming or molding temperature. The mold cavities and moldring will be hereinafter described in more detail.

The size and shape of the molds, mold ring and sleeve at roomtemperature (20 degrees C.) is then computed from the size and shapedata at forming or molding temperature as well as the relevant materialor composition parameters. As will be understood by one familiar withthe art, the preceding process is then repeated, as relevant, todetermine the master, master ring and master sleeve size and shape atboth the forming temperature and room temperature.

Usually, two molds and a lens ring are used for molding the lens. Eachmold forms one lens optical surface and the peripheral flat, while thering forms the lens barrel or the outer peripheral shape and size of thelens. As will be understood, some lenses do not require a lens flat andsome lenses may be molded without a ring.

One reason for forming a two piece master, snout and master ring, is toobtain a very small blend radius on the ultimate lens. The blend radiusis the radius between the curved optical portion of the lens and theperipheral flat portion. For many applications the required blend radiusis smaller that the tool radius that would be used to form it. Metalcutting tools having a diamond point with a 0.030" radius work well buttools with smaller radii are expensive, unreliable, and do not have along life. Therefore, to facilitate a more precision mold design and asmall blend radius, a two piece master design is preferred. It is to beunderstood that when a larger blend radius may be used or such a designis otherwise desirable, the master and ring may be fabricated in onepiece.

The optical cavity master shape is formed on surface 16 of snout 18 ofmaster 14. Such surface must be fabricated with precision and ispreferably formed by the single point diamond turning process (SPDT).This process is defined as using a single point on a diamond cuttingtool mounted in a precision lathe to machine the surface. An example ofa suitable lathe is an aspheric generator manufactured by Moore SpecialTool Co.

Maintaining close dimensional tolerances on the ring thickness and snoutheight, such as for example plus or minus 50 millionths of an inch,allows the ultimate mold cavity and mold flat to be precisely orientedproducing a relatively sharp corner as the blend radius. A clearance ofapproximately 0.001" is typically allowed between the outside snoutdiameter and the master ring inside diameter for acceptable operation.Of course, it will be understood that the tolerances described above areonly typical and may be varied depending on the materials andtemperatures involved, the precision of the lens desired and the utilityof the lens.

Slug or preform 10 is formed with a predetermined convex surface or bump24 as is additionally shown in FIG. 4. Bump 24 is disposed in opposingrelationship with surface 16 of snout 18. Surface 16 of snout 18together with surface 22 of master ring 20 combine to form the uppermold surface 26 including concave surface 28 of glass mold 30 as shownadditionally in FIG. 5. Although surface 22 is variously describedherein as flat or otherwise, it will be understood that it may containalignment or orientation marks and the like.

After slug or preform 10 is disposed in opposing relationship tosurfaces 16 and 22 of snout 18 and master ring 20 respectively withinsleeve 12, pushrod 32 is brought into position within sleeve 12 inoperative association with slug 10. A thermocouple, not shown, may beinserted into aperture 34 of master 14 as shown in FIG. 1 for measuringtemperature of master 14.

After apparatus 35 is thus assembled in a press, not shown, it is readyto be heated by any suitable means. The means of applying pressure inthe press is not critical and need only have a mounting means and a ramrod, and a pneumatic or hydraulic cylinder capable of exerting apressure sufficient to mold the particular molding composition at itsmolding temperature. The mold assembly and the mold material may beheated by any suitable means preferably by an RF induction field wellknow in the art. Such induction heating is preferred because the mastersleeve is a good susceptor, RF generators are readily availablecommercially, and good temperature control may be maintained. Heatingand pressing may be performed in a controlled inert atmosphere, such asa nitrogen glove box, to eliminate dirt, contaminants, and oxidation ofthe tooling, if desired.

The material for slugs 10 and 40 is not critical and the slugs may beformed of any suitable material depending on the ultimate material to bemolded, the temperatures involved, chemical compatibility and the like.One familiar with the art can select a suitable material for aparticular application. For forming a mold for the lens exampledescribed herein, a glass composition known as Schott F6 manufactured bySchott-Ruhrglas Gmbh of West Germany is suitable. This composition isabout as follows on an average weight percent: Si0₂ 42.7, PbO 46.8, BaO2.5, A1₂ O₃ 0.1, K₂ 0 5.4, Na₂ O 2.5. This glass has an annealing pointof about 425° C., a strain point of about 387° C., a nominal softeningpoint of about 591° C., and a coefficient of thermal expansion of about9 ppm/deg. (100°-300° C.).

The following is an example of a typical mold forming procedure usingthe Schott F6 glass described herein. The temperature of the moldingapparatus 35 with slug 10 in place as hereinabove described is raised toa soaking temperature of 510° C. by means of an RF induction field. Theassembly is soaked at 510° C. for about three minutes or until a thermalequilibrium of the assembly is reached. Thermal equilibrium is definedas a condition where the temperature variation across the slug is notmore than about 1 degree C.

Thereafter, a force is applied to pushrod 32 as shown by arrow 36. Aswill be well understood, pressure may be applied by means of a hydrauliccylinder press as described above. The force produced by the cylinder israised from about 48 lbs. to 148 lbs. in about 100 seconds, and theforce is maintained for about 7 minutes while the assembly is maintainedat about 510° C. Thereafter, the force is removed and the assembly isallowed to cool to about 400° C., that is a temperature at which themold material sets up.

The assembly is then further free cooled to about 150° C. and thefinished glass mold, such as that illustrated by reference numeral 30 ofFIG. 5, is removed from the mold assembly. A slug such as 10 of FIG. 4having a generally cylindrical shape with straight sidewalls is wellsuited for the lower or stationary mold as will be hereinafter describedin more detail.

The upper or movable mold 38, as illustrated in FIG. 3, may be formedfrom a slug 40 having a reduced diameter between shoulders 42 toultimately result in a glass mold 38 having raised bands 44 as shown inFIG. 3. These bands simulate the bearing bands on standard piston moldsand reduce the tendency for mold sticking during subsequent opticalelement pressing due to slight irregularities and/or dirt in the moldingsleeve. Otherwise, glass mold 38, having mold surface 46 and concavesurface 48, is formed from slug 40 having bump 50 in the mannerdescribed hereinabove in connection with the formation of glass mold 30.

Although the formation of the glass molds has been described in terms offorming a pair of glass molds to be used in a cooperative relationship,it is contemplated that only one glass mold may be formed as describedherein and disposed in an operative relationship, to form a moldingcavity, with a mold formed by other means and made of metal, glassceramic and the like, if and when desired. As will be understood, insuch an embodiment, the advantages of employing a molded glass mold asdescribed herein will be available only on one optical surface of theultimate optical element formed, however, depending on the opticalelement and the use to which it will be placed, that may besatisfactory.

The glass molds thus formed are annealed to relieve residual stresses.Such annealing may be done using equipment and procedures known in theart, and which varies with the composition of the materials beingannealed. However, care must be taken to carefully control the annealingprocess thereby controlling the amount of mold shrinkage in adeterminable manner as well as setting the fictive temperature.

An example of annealing the molds of the present lens example is asfollows. The glass molds are placed in an annealing oven manufactured bythe Blue M Electric Co. of Blue Island, IL., and the temperature israised at a rate of about 11 C. degrees per minute until the annealingtemperature of 430° C. is reached. That temperature is then maintainedfor about 30 minutes. The important cooling rate employed is about -0.6C. degrees per minute until a temperature of about 367° C. is reached.This is then followed by cooling at a rate of about -2 C. degrees perminute until a temperature of about 317° C. is reached. Thereafter themolds are force cooled to room temperature. The Blue M annealing ovenmay be programmed for the annealing cycle.

Other examples of glass compositions suitable for glass mold forming areCorning Glass Works glass composition codes 9012, 0120, and 8355. Itmust be understood that selection of a suitable glass mold compositiondepends on many variables including, importantly, the material to bemolded by such molds.

Referring now to FIG. 6 there is shown an apparatus for molding a glassoptical element such as lens 52 illustrated in FIG. 7. The moldingapparatus assembly comprises a sleeve post 54 disposed on a base 56.Element sleeve 58 is disposed about sleeve post 54 and the first mold 60is placed within sleeve 58 adjacent to sleeve post 54. The first moldingsurface 62, within which the molding configuration is formed, isdisposed within sleeve 58 in the direction away from sleeve post 54. Anelement ring 64 is then disposed within sleeve 58 on the surface of mold60. As will be understood, the inner opening of ring 64 will define theexterior configuration and size of the glass element to be molded. Innersurface 66 of ring 64 and the exposed portion of first molding surface62, exposed within the opening in ring 64, define the size andconfiguration of one side and the periphery of the glass element or lensto be molded.

An element preform 68 is then disposed on the first molding surface 62within ring 64. A second mold 70 is placed within element sleeve 58 suchthat the second molding surface 72 thereof comes in contact with elementpreform 68. As will be understood, the central portion of second moldingsurface 72 will ultimately form the opposite side of the glass elementor lens to be molded.

A pushrod 74 is then disposed over second mold 70 by means of which aforce as illustrated by arrow 76 may thereafter be applied. The force isapplied to the pushrod of the mold assembly by means of a hydraulic orpneumatic cylinder well known in the art. The force could also beapplied by means of dead weight or even through the weight of the moldif time is not important. It should also be understood that the presentinvention is described in terms of molding one glass lens or opticalelement but it is contemplated that multiple molding assemblies may beused at one time, wherefore, a means for applying a force to a pluralityof molding assemblies simultaneously, such as a hydraulic or pneumaticcylinder with a plurality of arms to contact each molding assembly, canbe used.

An lens or element preform 68 is first formed having the required volumeof the ultimate lens or glass element to be formed and a surface finishsuitable for forming the ultimate glass article. A spherical form forelement 68 is preferred for practical and economic reasons since boththe surface finish and the required volume may be readily controlled andobtained by conventional machining means which do not form part of thisinvention and will be readily understood.

A suitable glass composition for forming a glass lens 52 is an alkalialuminofluorophosphate optical glass having a composition as follows ona weight percent basis: P₂ O₅ 39.2, Na₂ O 5.0, F 4.3, PbO 24.2, BaO20.1, Li₂ O 2.0, and Al₂ O₃ 5.2. This composition has a strain point of330° C. A typical tolerance on the diameter of a spherical preform isplus or minus 0.008 mm and a typical finish is .LT. 1 microinch AA.Examples of other suitable compositions for forming glass opticalelements are examples 10 and 13 of U.S. Pat. No. 4,447,550, as well asother SiO₂ -B₂ O₃ -Pb0-Al₂ O₃ -F type glasses. As will be understood,the composition of the ultimate optical element or lens may vary greatlydepending on the use to which it may be put and/or the environment inwhich it is used. Therefore, the composition of the lens or opticalelement is not critical to and does not form part of the presentinvention. It will also be understood that the optical element or lensmaterial may be other than glass, as is herein described, and may be acombination of materials or various plastics.

A suitable oven for molding glass optical elements in accordance withthe present invention is an Ultratemp®, Model IGF 9980-4, manufacturedby Blue M Electric Co. of Blue Island, IL.

Typically, the oven is purged with an inert gas, such as nitrogen orargon, prior to use to prevent oxidation of the materials, equipment andapparatus employed. Purging for about 38 minutes with nitrogen at a gasflow of about 350-400 scfh has been found to be useful.

After the molding assembly is disposed in the oven and the oven ispurged, the oven heat-up cycle is started. The soaking-pressingtemperature of about 375° C. is reached in approximately 100 minutes. Atthe end of this heat-up soaking interval, the pressing cycle starts,typically for about 10-15 minutes, while the pressing temperature ismaintained. As will be understood, the pressing temperature depends onthe composition of the preform and its physical characteristics, such asthe softening point. It will also be understood that the force to beapplied by the pneumatic or hydraulic cylinder to the pushrod will varywith the preform material. For the alkali aluminofluorophosphate elementpreform material described heretofore, the pressing temperature will be375° C.

An important consideration in forming the optical element or lens isalignment of the mold parts. Specifically, the mold centerlines shouldbe coincident within predetermined limits for the particularapplication. Element sleeve 58 maintains the mold parts in alignment. Aprecision V-block could also be used for this purpose. The insidediameter of the element sleeve as well as all other dimensions aredetermined in the manner described in connection with the design of themaster sleeve hereinabove. The dimensional tolerance between the outsidediameter of the mold parts and the inside diameter of the sleeve areimportant for the desired axial alignment of the mold parts and must bedetermined for each application as herein described.

The element ring 64 is disposed intermediate molds 60 and 70 and alignedtherewith by element sleeve 58. Ring 64 dimensions are determined asdescribed in connection with master ring above taking into account thelens material characteristics and the ultimate lens configuration.

An important aspect of the pressing conditions selected for forming theglass optical element of the present invention is that the elementthickness is self-limiting as desired. Having formed the element preformand element ring as herein described, the process is operated in suchmanner as to produce an element thickness controlled by the resistanceof the formed glass optical element to further deformation. That is, ata specific temperature, the pressure used will be opposed by the forcerequired to cause physical deformation in the near final shape of theglass optical element being formed. Since the diameter is constrained byelement ring 64, the resultant molded article will have a finitethickness controlled by temperature and pressure. As will be understood,the process may also be operated at temperatures and pressures such thatelement ring 64 is the controlling factor for the thickness of theoptical element being formed.

After pressing is complete, the pressing cylinder is retracted, the ovendamper opens automatically, and an 80 minute oven cooling-off periodbegins. The entire cycle takes approximately 4 hours with the equipmentand materials described.

A typical example of forming a glass lens with glass molds is asfollows. Two glass molds corresponding to 60 and 70 are provided havinga desired molding surface configuration, an outside diameter of 0.6",and a length of 0 7". Also provided are an element sleeve 58 having aninside diameter of 0.6", an outside diameter of 0.8", and a length of2.25"; an element ring 64 having an inside diameter of 0.250", anoutside diameter of 0.6", and a thickness of 0.062"; and a sphericalpreform 68 having a radius of 2.494 plus or minus 0.0024 mm. Thedimensional tolerance between the outside diameter of the molds andelement ring and the inside diameter of the element sleeve is 0.0001".

The glass mold 60, fabricated by the process described earlier from saidSchott F6 optical glass, is placed within tungsten carbide sleeve 58.The tungsten carbide element ring 64 is then placed on the surface ofglass mold 60. Sleeve 58, glass mold 60 and element ring 64 are thenplaced over a type 304 stainless steel sleeve post 54 on a tool steelbase 56. Spherical element preform 68, formed of the alkalialuminofluorophosphate optical glass described above, having a radius of2.494 plus or minus 0.008 mm. and mass of 0.2470 plus or minus 0.0024gm, is placed within the formed element ring 64. A second glass mold 70is then placed within sleeve 58 in contact with element preform 68. Atype 304 stainless steel pushrod 74 is disposed within sleeve 58adjacent glass mold 70 thus completing a mold sub-assembly.

Additional sub-assemblies may be assembled on base 56 and alsosub-assemblies may be setup on additional base units, if desired. Inthis manner, a large number of optical elements or lenses may be moldedat one time greatly increasing productive efficiency. In such anembodiment, a plurality of arms are attached or affixed to, or otherwiseoperatively associated with, the pressing hydraulic or pneumaticcylinder so as to contact each sub-assembly and exert a force thereon asdescribed herein.

The base units are placed into a Ultratemp™ Inert Gas Oven, ModelIGF-9980-4, manufactured by Blue M Electric Company of Blue Island, IL.The inert gas oven is modified to accommodate a support framework forthe base units beneath a pressing rod connected to an external pneumaticcylinder The pneumatic cylinder is Model #D-16-F-SM-UM manufactured byBellowfram Co., SR2, Newel, W. Va. The oven is a forced gas circulationoven.

The oven is then closed and purged with N₂ gas for a period of about 40min. at a flow of about 350-400 scfh. At the end of the oven purge, theN₂ flow is lowered to about 90-110 scfh and maintained at that level toprovide an inert gas atmosphere during subsequent processing. The ovenis then heated to a temperature of about 375° C. and maintained at thattemperature by means of electrical heating elements.

After maintaining such temperature for about 100 minutes, the pneumaticcylinder is caused to contact the push rod; a pressure of 33 psi beingapplied to the cylinder. The pressure is transmitted to the moldassembly and maintained for about 10 minutes.

After pressing is completed, the cylinder pressing rod is retracted andthe oven set point controller temperature is lowered to about 25° C. Theoven damper is opened and external air circulation is started to allowfree cooling. Such free cooling is continued for about 80 minutes, afterwhich N₂ flow to the oven is stopped. The base units are then removedfrom the oven and glass optical elements or lenses 52 are removed fromthe sub-assemblies.

Glass optical elements 52 may then be annealed, if desired, by heatingto 259° C. at a rate of 2 C. degrees per minute; then heated to 329° C.at a rate of 1.15 C. degrees per minute; holding temperature at 329° C.for 5 minutes; then cooling to 225° C. at a rate of 0.86 C. degrees perminute; then cooling to 20° C. at a rate of 3.4 C. degrees per minute.

It has been found that forming the first and second glass molds inaccordance with the present invention results in enormous economies andbenefits for the following reasons. An ordinary metal mold has a typicalexpected life of about 300 cycles before it is attacked by the variouschemicals involved, scratched, distorted, or otherwise made unsuitablefor further precision molding. The life of the master of the presentinvention is approximately 30 cycles, but each glass mold formed from amaster can thereafter be employed in approximately 100 molding cycles.It is seen, therefore, that employing the glass molds of the presentinvention to further mold glass optical elements in accordance with thepresent invention, of a 10 fold economy is realized. That is, for eachmaster in accordance with the present invention, approximately 3000glass optical elements can be produced in comparison to 300 suchelements produced by employing prior art metal molds.

The present invention has been particularly shown and described withreference to preferred embodiments thereof, however, it will beunderstood by those skilled in the art that various changes in the formand detail may be made therein without departure from the true spiritand scope of the invention as defined by the following claims.

We claim:
 1. A method of forming a mold assembly including a pair ofglass molds defining first and second opposed optical molding surfacescomprising the steps of:providing a first master defining a first master mold cavity having a first predetermined master optical surfaceadapted to form a first glass mold, disposing a first quantity ofpreformed glass mold material within said first master mold cavity andheating said preform to a moldable temperature, press molding a firstglass mold within said first master mold cavity and impressing saidfirst master optical surface into said first glass mold material to forma first glass molding surface which is adapted to form a first finishedoptical surface, providing a second master defining a second master moldcavity having a second predetermined master optical surface adapted toform a second glass mold, disposing a second quantity of preformed glassmold material within said second master mold cavity and heating saidpreform to a moldable temperature, press molding a second glass moldwithin said second master mold cavity and impressing said second masteroptical surface into said second glass mold material to form a secondglass molding surface which is adapted to form a second finished opticalsurface, and said first and second glass molding surfaces whenpositioned in opposed operative relationship defining a glass moldcavity having a predetermined desired size, shape and volume.
 2. Themethod of claim 1 wherein said first and second master are formed ofmetal.
 3. The method of claim 2 wherein said first and second master areformed of a glass ceramic.
 4. The method of claim 2 wherein said firstand second glass molds are formed of a composition consistingessentially on a weight percent basis of SiO₂ 42.7, PbO 46.8, BaO 2.5,A1₂ O₃ 0.1, K₂ O 5.4, and Na₂ O 2.5.
 5. The method of claim 4 whereinsaid first quantity of glass comprises a glass slug which issubstantially cylindrical in shape and having a bump at one end thereof.6. The method of claim 5 wherein said second quantity of glass comprisesa glass slug which is substantially cylindrical in shape, having aprotruding shoulder at each end thereof along the cylindrical surface,and having a bump at one end thereof.
 7. The method of claim 1 furthercomprising the step of heating the first and second quantities of glassmold material to the softening temperature thereof.
 8. The method ofclaim 7 further comprising the steps ofproviding an oven, disposing saidfirst and second quantities of glass mold material in said oven, andmaintaining an inert gas atmosphere in said oven during said moldingsteps.
 9. The method of claim 8 wherein said inert gas is nitrogen. 10.The method of claim 9, further comprising the steps ofheating the firstand second quantities of said glass mold material and said respectivemasters to the molding temperature of said material, soaking said moldmaterial and masters at said molding temperature until thermalequilibrium is reached, and thereafter molding said glass molds.
 11. Themethod of claim 10, further comprising the step of annealing said glassmolds.
 12. The method of claim 8 wherein each said molding step furthercomprises applying a force to each assembly so formed
 13. The method ofclaim 1 wherein said first master comprises a snout portion and a masterring disposed thereabout forming said first master mold cavity.
 14. Themethod of claim 1 wherein each said molding step further comprisesapplying a force to each assembly so formed.
 15. The method of claim 1further comprising the step of soaking said first and second quantitiesof said glass mold material and said respective masters at the moldingtemperature of said material until thermal equilibrium is reached beforeeach said molding step.
 16. The method of claim 1 further comprising thestep of annealing said glass molds.
 17. The method of claim 1 furthercomprising the steps ofproviding an oven, providing and inert atmospherewithin said oven, disposing said first and second quantities of glassmold material and said respective masters within said oven, heating saidmold material and said masters to the molding temperature of saidmaterial, maintaining said inert atmosphere within said oven, soakingsaid mold material and masters at said molding temperature until thermalequilibrium is reached, applying a force to said mold material to effectmolding thereof, and annealing said first and second glass molds.
 18. Aglass mold assembly made by the method of claim
 1. 19. A method offorming a glass mold defining an optical molding surface comprising thesteps of:providing a master defining a master mold cavity having apredetermined master optical surface formed therein adapted to form aglass mold, disposing a quantity of preformed glass mold material withinsaid master mold cavity and heating said preform to a moldabletemperature, press molding a glass mold within said master mold cavityand impressing said master optical surface into said moldable glass moldmaterial to form a glass molding surface adapted to form a finishedoptical surface, and said glass molding surface at least in partdefining a glass mold cavity having a predetermined desired size, shapeand volume when positioned in opposed operative relationship with amolding surface of a second mold.
 20. The method of claim 19 whereinsaid master is formed of metal.
 21. The method of claim 20 wherein saidmaster is formed of a glass ceramic.
 22. The method of claim 20 whereinsaid glass mold is formed of a composition consisting essentially on aweight percent basis of SiO₂ 42.7, PbO 46.8, BaO 2.5, A1₂ O₃ 0 1, K₂ 05.4, and Na₂ O 2.5.
 23. The method of claim 22 wherein said quantity ofglass comprises a glass slug which is substantially cylindrical in shapeand having a bump at one end thereof.
 24. The method of claim 23 whereinsaid quantity of glass comprises a glass slug which is substantiallycylindrical in shape, having a protruding shoulder at each end thereofalong the cylindrical surface, and having a bump at one end thereof. 25.The method of claim 19 further comprising the step of heating saidquantitY of glass mold material to the softening temperature thereof.26. The method of claim 25 further comprising the steps ofproviding anoven, disposing said quantity of glass mold material in said oven, andmaintaining an inert gas atmosphere in said oven during said moldingstep.
 27. The method of claim 26 wherein said molding step furthercomprises applying a force to the assembly so formed.
 28. The method ofclaim 19 wherein said master comprises a snout portion and a master ringdisposed thereabout forming said master mold cavity.
 29. The method ofclaim 19 wherein each said molding step further comprises applying aforce to the assembly so formed.
 30. The method of claim 19 furthercomprising the step of soaking said quantity of said glass mold materialand said master at the molding temperature of said material untilthermal equilibrium is reached before said molding step.
 31. The methodof claim 19 further comprising the step of annealing said glass mold.32. The method of claim 19 further comprising the steps ofproviding anoven, providing and inert atmosphere within said oven, disposing saidfirst and second quantities of glass mold material and said respectivemasters within said oven, heating said mold material and said master tothe molding temperature of said material, maintaining said inertatmosphere within said oven, soaking said mold material and master atsaid molding temperature until thermal equilibrium is reached, applyinga force to said mold material to effect molding thereof, and annealingsaid glass mold.
 33. The method of claim 19 further comprising the stepof providing a second mold having a second molding surface adapted toform a second optical surface, said molding surface and said secondmolding surface in operative relationship defining a glass mold cavityhaving a predetermined desired size, shape and volume.
 34. The method ofclaim 33 wherein said second mold is formed of metal.
 35. The method ofclaim 37 wherein said second mold is formed of a glass ceramic.
 36. Themethod of claim 19 further comprising the step of providing a secondglass mold having a second glass molding surface adapted to form asecond optical surface, said glass molding surface and said second glassmolding surface in operative relationship defining a glass mold cavityhaving a predetermined desired size, shape and volume.